The present invention relates to a process for the production of di- and polyisocyanates of the diphenylmethane series and mixtures thereof, and to a process for the production of polyurethanes from the di- and polyisocyanates of the diphenylmethane series.
Aromatic isocyanates are very important raw materials for the production of polyurethane materials. In terms of quantity, the di- and polyisocyanates of the diphenylmethane series (MDI) play the leading role in this.
The term polyisocyanates of the diphenylmethane series means isocyanates and mixtures of isocyanates which correspond to the following structure:
where n denotes a natural number>2.
Similarly, the term polyamines of the diphenylmethane series means compounds and mixtures of compounds which correspond to the following structure:
where n denotes a natural number>2.
It is known that di- and polyisocyanates of the diphenylmethane series (MDI) are produced by phosgenation of the corresponding diamines and polyamines of the diphenylmethane series (MDA). The diamines and polyamines of the diphenylmethane series (MDA) are themselves produced by condensation of aniline and formaldehyde. By phosgenation of the diamines of the diphenylmethane series, the corresponding diisocyanates 2,2′-MDI, 2,4′-MDI and 4,4′-MDI are obtained, which are described in specialist circles as 2-ring (i.e. bi-nuclear) compounds of MDI (i.e. diisocyanates of the diphenylmethane series). During the condensation of aniline and formaldehyde, however, the 2-ring MDA (i.e. methylenediphenyldiamine) continues to react further with formaldehyde and aniline to form multi-ring MDA types, which, after phosgenation, represent the multi-ring (i.e. polynuclear) content in the polymeric MDI (polyisocyanates of the diphenylmethane series).
For many practical product applications, it is preferred that a defined acid content be established in the MDI. This standardization compensates for any variations in reactivity caused by secondary compounds in the reaction chain from aniline to MDI. These secondary compounds have an influence on the acid content of the MDI during processing with compounds which contain isocyanate-reactive hydrogen atoms, such as, for example, the polyols used in polyurethane chemistry.
In practice, the acid content in MDI is expressed as acidity in polymeric MDI (PMDI) and/or monomeric MDI (MMDI), or as hydrolysable chlorine in MMDI. This acidity value is determined, in practice, by reacting MDI with lower alcohols such as, e.g. methanol (cf. e.g. ASTM D5523-94 for monomeric MDI or ASTM5629-99 or 6099-03 for polymeric MDI).
The acid content in MDI is determined, as described in the current state of the art, by the processing parameters in the production process for the MDA bases by the formation of secondary components in the MDA, which is dependent on the relevant MDA process parameters, and during the phosgenation of MDA to MDI, and subsequent work-up. Because of the multi-step reaction chain and the variations and changes in the throughput and process parameters during production, this leads in practice to unavoidable variations in the acid content. Values for the acid content in the MDI as MDI acidity of up to 500 ppm acid (calculated as HCI) are generally achieved. However, deviations from the desired acidity sometimes occur. These variations in acidity within the product quality of a manufactured MDI result in undesirable deviations in the reaction behavior of the MDI during the subsequent polyurethane production.
In general, it is correct that as the acid content increases, the reactivity of MDI (i.e. the relative reactivity of isocyanate to polyol) slows down or decreases. However, since during polyurethane processing the reactivity should generally be defined by the catalysis in the polyol, it is essential for safe and reproducible processing for the MDI component to have constant, and consistent, reactivity. The acid content achieved during production as described in the prior art, and particularly in the case of MMDI (monomeric MDI), can even be so low such that undesirable secondary reactions occur during polyurethane processing (e.g. alkali-catalysed polyisocyanurate reaction).
In the marketplace, therefore, the various MDI products commercially available are provided with a specification value for the acidity or the hydrolysable chlorine content (i.e. HC value). The analyses of the acidity are generally performed according to the ASTM test methods mentioned above, which describe the content of acid released during reaction with compounds which contain active hydrogen atoms. This value is generally calculated as HCl.